Stormwater is rainwater or snowmelt that runs off impervious surfaces like roofs, roads, sidewalks, and parking lots rather than soaking into the ground. Urban development increases stormwater runoff by replacing natural surfaces with impervious materials. Stormwater picks up pollutants from surfaces—oil, fertilizers, sediment, bacteria, and debris—carrying them to waterways and degrading water quality. Managing stormwater involves infrastructure (storm sewers, detention ponds) and increasingly green infrastructure (rain gardens, permeable pavements, green roofs) that slow, absorb, and filter runoff. Climate change intensifying storm events increases stormwater management challenges. Stormwater management is a growing municipal priority and cost, driving investment in infrastructure and development requirements for on-site stormwater management.